I am unable to copy from vim to system clipboard. Viceversa works, copying from outside vim and pasting with p is ok.

I have installed clipboard support, vim version is 7.3.429:

$ vim --version | grep clip

+clientserver +clipboard +cmdline_compl +cmdline_hist +cmdline_info +comments 

+xsmp_interact +xterm_clipboard -xterm_save 

I have:

set clipboard=unnamedplus

in my .vimrc and it is set when I do:

set clipboard?

or

:echo has('clipboard')

I am using Ubuntu 12.04 LTS, and vim inside gnome-terminal (but also GVim has the same problem). Any idea?